﻿PTCNODONTID.E. 
  191 
  

  

  with 
  the 
  median 
  ridge 
  on 
  the 
  superior 
  aspect 
  of 
  the 
  coalesced 
  vomers 
  : 
  

   it 
  can 
  be 
  distinguished 
  from 
  this 
  plate 
  by 
  the 
  arrangement 
  of 
  its 
  

   structural 
  fibres, 
  which 
  are 
  directed 
  downwards 
  and 
  forwards 
  

   instead 
  of 
  horizontally, 
  and 
  it 
  never 
  projects 
  in 
  advance 
  of 
  the 
  

   vomers 
  in 
  the 
  rostrum. 
  There 
  is 
  no 
  ossified 
  interorbital 
  septum. 
  

   It 
  is 
  evident 
  from 
  several 
  specimens, 
  notably 
  Anomceodus 
  willetti 
  

   and 
  Mesturus 
  leedsi, 
  that 
  the 
  base 
  of 
  the 
  cranium 
  bears 
  a 
  delicate, 
  

   laminar, 
  median 
  vertical 
  keel 
  extending 
  as 
  far 
  forwards 
  as 
  the 
  orbit 
  ; 
  

   but 
  the 
  only 
  precise 
  information 
  concerning 
  the 
  basicranial 
  axis 
  has 
  

   been 
  obtained 
  from 
  specimens 
  of 
  Mesturus 
  leedsi. 
  The 
  basiocci- 
  

   pital 
  is 
  unknown, 
  but 
  a 
  detached 
  element, 
  evidently 
  chondrocranial, 
  

   in 
  no. 
  P. 
  6834 
  recorded 
  below, 
  fits 
  well 
  on 
  the 
  hinder 
  margin 
  of 
  

   another 
  bone 
  in 
  the 
  same 
  fossil 
  which 
  is 
  proved 
  to 
  be 
  parasphenoid. 
  

   It 
  may 
  therefore 
  be 
  determined 
  as 
  basisphenoid, 
  and 
  is 
  noteworthy 
  

   as 
  occupying 
  a 
  considerable 
  length 
  of 
  the 
  basicranial 
  axis. 
  It 
  is 
  

   longer 
  than 
  broad, 
  exhibits 
  a 
  median 
  longitudinal 
  keel 
  below, 
  and 
  

   is 
  narrowest 
  at 
  its 
  hinder 
  end 
  ; 
  while 
  the 
  lateral 
  margins 
  form 
  

   sharp 
  edges, 
  each 
  apparently 
  notched 
  near 
  its 
  anterior 
  end, 
  and 
  a 
  

   pair 
  of 
  large 
  foramina 
  pierce 
  the 
  bone 
  obliquely 
  near 
  its 
  middle. 
  

   The 
  parasphenoid 
  is 
  remarkable 
  for 
  its 
  shortness, 
  not 
  extending 
  

   backwards 
  beyond 
  the 
  front 
  edge 
  of 
  the 
  supposed 
  basisphenoid. 
  

   The 
  main 
  part 
  of 
  the 
  bone 
  is 
  short, 
  stout, 
  and 
  broad, 
  with 
  a 
  median 
  

   longitudinal 
  keel 
  below 
  ; 
  and 
  its 
  basipterygoid 
  processes 
  are 
  short, 
  

   with 
  the 
  very 
  coarsely 
  dentated 
  articular 
  end 
  already 
  mentioned. 
  

   At 
  the 
  front 
  extremity 
  of 
  the 
  otic 
  region 
  this 
  broad 
  bone 
  is 
  abruptly 
  

   truncated, 
  its 
  inferior 
  keel 
  terminates 
  in 
  a 
  roughened 
  pointed 
  knob, 
  

   and 
  the 
  element 
  continues 
  merely 
  as 
  a 
  narrow 
  slender 
  median 
  bar 
  

   across 
  the 
  region 
  of 
  the 
  orbit 
  ; 
  towards 
  the 
  front 
  of 
  the 
  orbit 
  in 
  

   many 
  fossils 
  a 
  vertical 
  median 
  lamina 
  is 
  observed 
  to 
  rise 
  gradually 
  

   on 
  its 
  superior 
  face 
  to 
  meet 
  the 
  ascending 
  plate 
  of 
  the 
  coalesced 
  

   vomers. 
  The 
  latter 
  form 
  one 
  very 
  stout, 
  bilaterally 
  symmetrical 
  

   bone 
  extending 
  to 
  the 
  anterior 
  end 
  of 
  the 
  rostrum, 
  where 
  they 
  

   are 
  overlapped 
  by 
  the 
  small 
  premaxillae. 
  

  

  The 
  arrangement 
  of 
  the 
  membrane-bones 
  (or 
  dermal 
  plates) 
  of 
  

   the 
  cranial 
  roof 
  is 
  not 
  quite 
  constant, 
  and 
  there 
  is 
  much 
  irregularity 
  

   especially 
  in 
  the 
  superficial 
  elements 
  covering 
  the 
  ethmoidal 
  region. 
  

   As 
  shown 
  by 
  Mesturus 
  (fig. 
  30), 
  the 
  posterior 
  median 
  element 
  of 
  the 
  

   roof 
  is 
  bilaterally 
  symmetrical 
  and 
  unpaired, 
  and 
  may 
  therefore 
  

   be 
  named 
  supraoccipital 
  (s.occ). 
  The 
  frontals 
  (jr.) 
  are 
  the 
  largest 
  

   plates, 
  always 
  conspicuous 
  and 
  readily 
  identified 
  between 
  the 
  orbits, 
  

   and 
  varying 
  in 
  length 
  according 
  to 
  the 
  shape 
  of 
  the 
  head. 
  The 
  

   otic 
  region 
  is 
  covered 
  by 
  a 
  single 
  plate, 
  which 
  forms 
  the 
  postero- 
  

   lateral 
  angle 
  of 
  the 
  roof 
  and 
  meets 
  the 
  frontal 
  anteriorly, 
  entering 
  

  

  